An approach to performing Second Harmonic Generation (SHG) with low-power lasers is to confine the fundamental power over long distances in an optical waveguide. A problem of producing a high-quality optical waveguide of sufficient nonlinearity for SHG or parametric amplification in principle may be overcome with poled polymers. As discussed by G.T. Boyd in journal article "Applications requirements for nonlinear-optical devices and the status of organic materials", J. Opt. Soc. Am. B, Vol. 6, No. 4, 4/89 a major application of such a SHG poled polymer is frequency doubling of laser diodes from approximately 800 to 400 nm, thereby improving data storage packing densities and data capture rates.
As described by Boyd the expression for the output power is essentially identical to that for single crystal SHG, except for a factor, S, referred to as the overlap integral: ##EQU1## where E.sub.m (.omega.,z) is the waveguide field of frequency .omega. and mode m, and where y and z are the spatial dimensions in the plane perpendicular to the propagation direction. One technique to phase match SHG in the waveguide is to use different modes (m and m') for the fundamental and the second harmonic such that effective waveguide index n.sub.m (.omega.)=n.sub.m,(2.omega.). However, if the fundamental and the second harmonic are confined to the same guiding channel, the two different modes generally have a small overlap integral. As an example, a phase-matched conversion from TE.sub.o (.omega.) to TE.sub.2 (2.omega.) gives a value for S of approximately 0.2%. For an input power of mW at 810 nm, and waveguide dimensions of 0.5 m.sup.2 .times.1 cm, one finds that a second order nonlinear susceptability .chi..sup.(2) .gtoreq.3.times.10.sup.-8 esu is required for a 1-mW output. However, if perfect phase matching and mode overlap can be achieved, the necessary nonlinearity is reduced to .chi..sup. (2) .gtoreq.2.times.10.sup.-9 esu.
In U.S. Pat. No. 4,865,406, issued Sept. 12, 1989 entitled "Frequency Doubling Polymeric Waveguide", G. Khanarian and D.R. Haas describe SHG with a polymeric waveguide having a periodic structure for quasi-phase matching of propagating laser energy. Poling of the thin film polymeric waveguide medium is achieved by heating the medium near or above its melting point or glass transition temperature, by example 90.degree. C., and applying a DC electric field of 50-150 V/micron to align molecular dipoles in a uniaxial direction. The medium is subsequently cooled with the field still applied to immobilize the aligned molecules within poled domains. Upper and lower poling electrodes are required, at least one of the electrodes having a grating configuration.
FIG. 1 illustrates a thin film waveguide similar to that illustrated in U.S. Pat. No. 4,865,406 wherein a substrate supports a nonlinear optically active polymer film having a periodic nonlinear optical modulation zone. A portion of input laser radiation of wavelength lambda is frequency doubled to provide output radiation of wavelength lambda/2. Prisms are employed to couple the radiation into and out of the waveguide.
One perceived problem with this teaching of Khanarian et al. relates to the relatively high temperature processing step required to raise the polymer above its melting point or glass transition temperature. As a result of the significant thermal energy present in the polymer a significantly larger magnitude DC potential is required to align and maintain in alignment the molecules than would be required if this processing step were accomplished at, for example, room temperature. Also, the use of a large magnitude DC electric field presents a danger of arc-over and degradation or destruction of the polymer film and in any event places a lower limit on the thickness of the waveguide structure. Furthermore, high temperature processing may be inconsistent with other materials if the waveguide forms a part of a composite structure.
Another perceived problem with this teaching is the requisite fabrication of the small geometry grating electrode structures for generating a periodic electric field for forming the corresponding poled domains. The grating electrode fabrication steps entail several processing steps including photolithographic and etching processes such as those employed for the fabrication of integrated circuits. As a result, processing complexity is increased and device cost is adversely impacted.
A still further perceived problem with this teaching relates to a resultant nonoptimum shape or geometry of the poled domains within the periodic modulation zone. It can be shown that a sine function represents an optimum cross-sectional domain shape to reduce or eliminate wasted Fourier energy and energy losses at other spatial frequencies. However, it is believed that the grating electrodes of Khanarian et al. will not impart such an optimum shape to the poled domains and as a result the waveguide of Khanarian et al. experiences energy inefficiencies.
In U.S. Pat. No. 4,818,070, Apr. 4, 1989, entitled "Liquid Crystal Optical Device Using U.V.-Cured Polymer Dispersions and Process for its Production" Gunjima et al. disclose a liquid crystal device cured with ultraviolet (UV) radiation. A voltage is applied during curing to optically orient at least a portion of the liquid crystal material. This disclosure of Gunjima et al is limited the modification of a refractive index of a portion of a liquid crystal material and does not extend to optical waveguides for SHG.
In U.S. Pat. No. 4,187,265, Feb. 5, 1980, entitled "Method of Making Ornamental Plastic Product" by D. Fischler there is disclosed a resinous mass containing lamella that is exposed to an orienting influence, such as contact with a roller, before curing The mass is said to preferably be cooled prior to orientation and irradiation with an electron beam in order to increase the viscosity of the mass. Increased viscosity is said to permit more efficient and complete orientation with the application of less orientating forces and to reduce a tendency toward disorientation (col. 8, lines 48-63).
U.S. Patents of general interest include the following. U.S. Pat. No. 4,182,790, Jan. 8, 1980, entitled "Liquid Alkylacrylamides and Related Compositions" by Schmidle discloses radiation curable compositions. U.S. Pat. No. 4,191,622, Mar. 4, 1980, entitled "Apparatus and Method for Producing Stereo-Regular Polymers" by Phillips et al. teaches the production on an electric field across a light-activated monomer in order to orient the monomer. U.S. Pat. No. 4,424,252, Jan. 3, 1984, entitled "Conformal Coating Systems" by L. Nativi discloses a UV curable conformal coating system and U.S. Pat. No. 4,734,143, Mar. 29, 1988, entitled "Process for the Production of a Continuous Composite Ribbon Including an Acrylate Resin Film to be used in Safety Laminated Glass Panels" by M. Meoni discloses the production of safety glass by a two step irradiation of a polymerizable mixture.
Other U.S. Patents of general interest dealing with aspects of curable compositions include the following: U.S. Pat. No. 3,637,419, Jan. 25, 1972 entitled "Method of Coating Rigid Cores and Product Thereof" by C. Lundsager, U.S. Pat. No. 3,661,614, May 9, 1972, entitled "Radiation-Curable Ink Compositions" by Bassemir et al., U.S. Pat. No. 4,165,265, Aug. 21, 1979, entitled "Multi-Stage Irradiation Method of Curing a Photocurable Coating Composition" by Nakabayashi et al. and U.S. Pat. No. 4,099,859, July 11, 1978, entitled "Contact Lens Having a Smooth Surface Layer of Hydrophilic Polymer" by E. Merrill.
However, none of these U.S. Patents disclose methods or apparatus suitable for fabricating an optical waveguide structure that overcomes the foregoing problems and which realizes other advantages and efficiencies.
It is thus an object of the invention to provide methods for fabricating a structure having a region characterized by regularly repeating, optically modulating domains, the methods not requiring a high temperature processing step.
It is another object of the invention to provide methods for fabricating a frequency doubling polymeric waveguide structure that does not require a high temperature processing step.
It is a further object of the invention to provide methods for fabricating a frequency doubling polymeric waveguide structure accomplished without a requirement of first fabricating a periodic electrode structure.
It is one further object of the invention to provide methods for fabricating a frequency doubling polymeric waveguide structure having poled domains of optimum shape for minimizing optical losses.
It is a still further object of the invention to provide methods for fabricating a thin film polymeric structure having a region characterized by regularly repeating domains containing noncentrosymetric dye molecules poled into a desired orientation and fixed in position by photopolymerization.
It is one further object of the invention to provide optically active devices constructed in accordance with the foregoing objects of the invention.